The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably a colored dielectric solvent. This type of display was first proposed in 1969. An electrophoretic display typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between them. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of electrophoretic displays, row and column electrodes on the top and bottom plates, respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom substrate and a common, non-patterned transparent conductor plate on the top viewing substrate may be used for the active type electrophoretic displays.
An electrophoretic dispersion composed of a dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrode plates. When a voltage difference is imposed between the two electrode plates, the charged pigment particles migrate by attraction to the plate of polarity opposite that of the charged pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the electrode plates, may be either the color of the solvent or the color of the charged pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing times.
Electrophoretic displays of different pixels or cell structures have been reported previously, for example, the partition-type EPD [M.A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)], the microencapsulated electrophoretic display (U.S. Pat. Nos. 5,961,804 and 5,930,026 and U.S. applications, Ser. No. 60/443,893, filed Jan. 30, 2003 and Ser. No. 10/766,757, filed on Jan. 27, 2004, now U.S. Pat. No. 7,184,197) and the total internal reflection (TIR) type of electrophoretic display using microprisms or microgrooves as disclosed in M.A. Mossman, et al, SID 01 Digest pp. 1054 (2001); SID IDRC proceedings, pp. 311(2001); and SID'02 Digest, pp. 522 (2002).
An improved electrophoretic display technology was disclosed in U.S. Pat. No. 6,930,818 (corresponding to WO01/67170), U.S. Pat. No. 6,672,921 (corresponding to WO02/01281) and U.S. Pat. No. 6,933,098 (corresponding to WO02/02/65215), the contents of all of which are incorporated herein by reference in their entirety. The improved electrophoretic display comprises isolated cells formed from microcups and filled with charged pigment particles dispersed in a dielectric solvent or solvent mixture. To confine and isolate the electrophoretic dispersion in the cells, the filled microcups are top-sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof. Most significantly, the microcup technology has made roll-to-roll manufacturing of electrophoretic displays, even large size electrophoretic displays, possible.
However, currently there are no commercially available low cost high resolution backplanes for driving large size electrophoretic displays. In addition, both direct drive with a dot matrix patterned electrode and active matrix drive with a TFT backplane need driving electronics that are too expensive for large size displays.
Mechanically movable electrode is considered one of the possible cost effective ways for the large size displays. Addressing an electrophoretic display with the combination of a stationary common electrode and a movable electrode to generate a local electric field was proposed by Chiang et al (see “A Stylus Writable Electrophoretic Display Device”, SID 1979 Digest, page 44). The mechanism worked well in theory, but ran into difficulty in practical applications. The main obstacle is the poor electrical contact between the movable electrode and the surface of the electrophoretic display panel. When the movable electrode is in contact with the surface of the electrophoretic display panel, due to rigidity and surface roughness of both the movable electrode tip and the electrophoretic display panel, the interface is dominated by air pockets and as a result, there is very limited true contact area. The poor contact results in a significant loss of electric field intensity at the contact interface and diminishes the electric field intensity across the electrophoretic display panel. Typically, when-two stationary electrodes are used, an electrophoretic display panel can be driven at a field intensity of less than 1 V/um. However, when a movable electrode is used to replace one of the stationary electrodes, a much higher overall electric field intensity, over 20 V/um, may be needed to achieve any reasonable image change in the same electrophoretic display set-up.
One way to avoid this poor electric contact is to change the movable contact surface from conductive/dielectric to conductive/conductive. For example, in the latter scenario, a printed circuit board (PCB) can be used. The printed circuit board, with dot matrix conductive pattern on both sides that are connected through via holes, is laminated onto the electrophoretic display panel. The lamination secures the contact of one conductive surface of the printed circuit board with the dielectric surface of the electrophoretic display panel. The movable electrode, in this case, is in contact with the conductive pads on the other conductive surface of the printed circuit board. Since both of the contact surfaces are conductive, the contact resistance is negligible. Alternatively, conductive pads may be directly pre-deposited onto the electrophoretic display panel to form a secured bond between the pads and the display panel. The movable electrode is then in contact with the conductive pads to generate an electric field. However, in these methods, the resolution of the image is inevitably limited by the resolution of either the printed circuit board or the pre-deposited conductive pads.